Premature detonation, pre-ignition, or combustion knock, occurs in internal combustion engines when the air/fuel mixture is being compressed by the engine cylinder and the compression temperature causes auto-ignition of the air/fuel mixture prior to ignition of the spark plug for that cylinder. Combustion knock can also occur when some or all of the air fuel mixture in the combustion chamber auto-ignites, or detonates, due to excessive in-cylinder compression temperature. Such knocking can cause severe damage to an engine as the pressure wave from the knock destroys the cylinder thermal boundary layer, causing the cylinder and piston to reach temperatures near the actual temperature of combustion. In an engine, this elevated temperature, combined with the pressure wave from the knock, can blow chunks off of the piston crown, which has a melting point much lower than the temperature of combustion. Even in engines with iron cylinder liners which can withstand the elevated temperatures, the piston rings in contact with the liners cannot withstand these temperatures and will fail. Allowing an engine to operate under a knocking condition can therefore lead to catastrophic engine damage.
Natural gas fueled engines are very prone to pre-ignition or detonation damage from a variety of means, such as overfueling, variation in fuel quality (i.e., high BTU fuel), elevated compression ratio due to combustion chamber deposit buildup, too much spark timing advance, etc. Before electronically controlled engines were developed, this problem was compensated for with low BMEP (power density) ratings and low compression ratios. This resulted in large engines with very low power densities, which translated into high initial costs and high continuous operating costs.
In order to achieve small package sizes and good operating efficiencies, modern electronically-controlled engines are typically designed as high BMEP, lean burn engines which have comparatively low detonation margins. It is generally considered imperative that such engines utilize a knock sensor in order to shut down the engine if dangerous knocking occurs.
Standard automotive gasoline engine knock protection systems generally utilize a single sensor mounted on the engine block in order to detect knocking in any engine cylinder. Such sensors typically sense vibration energy transmitted to the engine block from the premature detonation in any engine cylinder. Alternatively, such sensors can be mounted on the engine cylinder head in these engines. Such a knock sensor mounting arrangement is not feasible on natural gas fueled engines which utilize wet sleeve replaceable cylinder liners. In this system, the water jacket of the engine block is in contact with the outer surface of the cylinder liners and the contact points between the cylinder liners and the engine block have rubber seals therebetween. These seals and the engine water jacket effectively dampen the vibrations produced by premature detonation, such that these vibrations cannot be adequately sensed by a single sensor placed on the engine block or cylinder head. Furthermore, such engines typically have individual cylinder heads for each cylinder, further isolating the vibration energy transmission paths.
In order to provide knock detection on such natural gas fueled engines, prior art systems typically utilize a single knock sensor for each individual cylinder, typically mounted on the cylinder head. Such a design translates to relatively high cost and large space requirements for these systems. There is therefore a need for a knock sensor system for natural gas engines with wet sleeve replaceable cylinder liners that does not require a knock sensor for every engine cylinder. The present invention is directed toward meeting this need.